The fabrication of integrated optical devices demands very accurate technological methods to cope with very restrictive tolerances in the chemichal, physical and geometric characteristics of the manufactured devices. In fact even small errors can cause considerable variations in the required optical characteristics, e.g. a different spectral interval of operation, different splitting ratios in beam splitters or in directional couplers, and the like. These disadvantages can derive not only from inaccuracies in successive steps of the technological process of fabrication, but also from errors or insufficiencies of the design techniques, especially when all the boundary conditions and their possible variations were not taken into account.
Since the guiding structures forming the integrated optical devices are expensive and difficult to produce (e.g. in case of semiconductor materials, from special crystals, and the like) manufacturing rejects highly affect the final cost. Hence it can be convenient to adjust the optical characteristics of the obtained device to return its performance to the required specifications.
If the device is made of electrical-field-sensitive material, e.g. of lithium niobate (LiNbO.sub.3), or of certain semiconductor materials, its optical characteristic modification can be obtained by using the method described in the paper entitled "Waveguide Electrooptic Modulators" by Rod C. Alferness, issued on page 1121 of IEEE Transactions on Microwave Theory and Techniques, August 1982.
According to this method suitable electrodes are placed on the device for applying a potential difference of convenient value. The potential difference generates an electrical field permitting a change in the optical characteristics of the device in its operation phase. The method requires further steps in the technological process of fabrication to manufacture the electrodes and, once the device is fabricated, it requires permanent application of a voltage of controlled value, even though the operation conditions are always the same.